One example of a multiple access method of a digital radio communication system is CDMA (Code Division Multiple Access) system. 3GPP (3rd Generation Partnership Project) which is a standard association of the mobile radio communication system adopts, in its standardization, the CDMA system wherein a DSCH (Downlink Shared CHannel) used for high-speed data communication in a downlink (link from a base station to a communication terminal) is defined as one of the common channels shared by a plurality of communication terminals.
Since in each communication terminal, the DSCH is allocated to be used in a predetermined transmission unit (for example, each one frame), then it is expected to be utilized for high-speed packet communication, etc., on a downlink.
An explanation for a communication procedure of high-speed packet communication using DSCH channel is given below. FIG. 1 is a conceptual diagram of the high-speed packet communication system using DSCH and FIG. 2 is a diagram showing the relationship between a conventional communication procedure of the high-speed packet communication using DSCH and the receiving SIR at a communication terminal.
In high-speed packet communication using DSCH, a base station BS compares SIR (Signal to Interference Ratio) information notified by individual communication terminals MS#1 to MS#n as shown in FIG. 1 and assigns DSCH to a communication terminal which has the best channel condition (here, MS#1) so as to transmit DSCH data (namely, high-speed packet data).
Further, FIG. 2 shows the communication procedure during the time of measurement of receiving SIRs at individual communication terminals up to transmission of DSCH data to the respective communication terminals. Specifically, during t0 to t1, the communication terminals MS#1 to MS#n measure SIR for one frame of the downlink DPDCH (Dedicated Physical Data CHannel). Each communication terminal executes the averaging of the SIRs measured in every slot of the DPDCH data over one frame.
At next frame (from t1 to t2), the communication terminals MS#1 to MS#n notifies the one frame average SIR to the base station BS using uplink DPDCH (link from a communication terminal to a base station).
At the third frame (from t2 to t3) the base station BS compares the average SIRs notified from the respective communication terminals and determines, as a destination of DSCH data transmission, a communication terminal which has the best channel condition, i.e., a communication terminal which has the largest SIR (here, MS#1). Moreover, the base station BS determines a modulating, and coding schemes in addition to transmitting power (hereinafter, they are referred to as “transmission system”) as a transmission method of the DSCH data according to the SIR notified from the communication terminal assigned as a destination of DSCH data. In other words, the base station BS determines modulating and coding schemes in addition to transmitting power which were optimal for the channel condition during the past time t0 to t1 as the transmission system of the DSCH data. The optimal transmission system is determined in advance according to an SIR level.
At the last frame (from t3 to t4), the base station BS transmits the DSCH data using downlink DSCH to the communication terminal assigned as a transmission destination according to the determined transmission system.
However, when a high-speed packet communication is carried out according to the procedure shown in FIG. 2, there will be two frames period (from t1 to t3) between the first frame (from t0 to t1) at which the respective communication terminals measure SIRs of DPDCH data and the last frame (from t3 to t4) at which the base station BS transmits DSCH data. Therefore, within this two frames period (from t1 to t3), the propagation environment mostly will be changed, and hence, it is possible that the SIR of DSCH data received by a communication terminal is greatly deteriorated. That is to say, at a communication terminal, there is a possibility that SIR of DSCH received during t3 to t4 is greatly deteriorated in comparison with the SIR of DPDCH data received during t0 to t1.
Since the base station BS has determined the modulating, and coding schemes as well as the transmitting power as the transmission system of DSCH data which are optimal for SIR measured from t1 to t2 and if SIR notified from a communication terminal is different from the SIR of DSCH data, then, the transmission system of DSCH data is not always optimal. Therefore, there is a possibility that the quality of DSCH data is deteriorated.